1. Field of the Invention
The present invention is drawn generally to electrochemical fluid delivery devices, and specifically to an improved electro-osmotic fluid delivery system.
2. State of the Prior Art
Fluid delivery devices are well known in the art, ranging from pressurized fluid delivery, to mechanical fluid delivery, to electrochemical fluid delivery devices and beyond. One particularly interesting fluid delivery system is an electro-osmotic cell coupled with a delivery pump, forming an electro-osmotic pump. These simple pumps operate through the combination of an electrochemical cell and an ion-selective membrane to create a driving force for fluid delivery.
Conventional electro-osmotic pumps, however, have a number of problems that have not, as of yet, been addressed in the prior art. One particular problem has occurred in constant fluid delivery applications. As the operation of the device is continued over a period of time, it has been observed that the delivery rate is inconsistent, even though the current rate between the anode and the cathode is maintained at a constant rate. Generally, two types of osmosis are occurring with an electro-osmotic cell simultaneously. The primary and most prevalent type of osmosis is electro-osmosis, whereby charged ions (dissociated salts) are driven across an ion exchange membrane as the cell is operated, thereby dragging water molecules with them. The secondary and less prevalent form of transport is osmosis due to environmental conditions. Osmosis is the transfer of a solvent across a barrier, generally from an area of lesser solute concentration to an area of greater solute concentration. Given normal cell operating conditions, the environmentally-driven osmosis is negligible in comparison to the electro-osmosis.
As the relative concentrations of salts within the half cells of an electro-osmotic delivery device change, however, significant changes in the amount of fluid delivered have been observed. It has been postulated that as operation of the device is continued, the passage of ions (salts) across the membrane of the device causes an increase in the salt concentration within one of the half-cells. Eventually, the concentration becomes so high that a significant environmental osmosis flux develops. Thus, environmental osmosis becomes more prevalent, and affects the predictability and reliability of the cell operations. The additional solvent transfer causes an increase in the overall fluid amount contained in the one half-cell, increasing the rate of delivery of fluid.
The above-described effect can continue even after the operation of the cell has stopped. Even though the anode and the cathode are removed from electrical communication with one another, the concentration difference between the half-cells remains. Thus, additional solvent will continue to be pulled across the membrane, causing the fluid delivery device to continue delivering fluid even after the cell has ceased operation. This additional fluid delivery is termed “zero-current transport,” and is deemed unacceptable—especially for long term use of a constant-rate fluid delivery device.
It is thus an object of the present invention to provide an improved cell design wherein the concentration differences between the half-cells within the device are avoided.
It is another object of the present invention to increase the reliability and consistency of the delivery rate of the device.
It is a further object of the present invention to eliminate, or substantially reduce, unwanted zero-current transfer.
These and other objects will become apparent to one of ordinary skill in the art in light of the present specification, claims and drawings appended hereto.